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JAKE TAPPER, CNN ANCHOR: The search is on for a possible ISIS terror cell.

THE LEAD starts right now.

ISIS claiming responsibility, while police identify the terrorist who killed 22 innocent people at an Ariana Grande show in England. Today, we hear the harrowing stories from parents desperately searching for their children amidst the chaos.

President Trump calling those behind this attack evil losers before landing in Europe to meet with the pope.

Plus, enough to make the former CIA director worried. John Brennan testifies about the Trump campaign's contact with the Russians, but does he see any concrete evidence of collusion?

Good afternoon, everyone. Welcome to THE LEAD. I'm Jake Tapper.

We are going to begin today with our world lead.

At least 22 innocent people, including children as young as 8, have been killed in the deadliest terror attack in Britain since the 2005 London bombings. Nearly 60 more people have been injured.

The attack happened in Manchester, which is about a four-hour drive north of London, as an Ariana Grande concert was ending at the Manchester Arena. Panic and chaos ensued after the blast, which was near the box office part of the arena complex.

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UNIDENTIFIED FEMALE: Oh, my gosh. What's going on?

UNIDENTIFIED FEMALE: What just happened?

UNIDENTIFIED FEMALE: What's going on? Oh, my God.

(END VIDEO CLIP)

TAPPER: Police say that they believe as of now a lone suicide bomber, 22-year-old Salman Abedi, carried out the attack. Authorities are now trying to determine if Abedi was part of a larger network, while ISIS has claimed responsibility for the deadly bombing.

We have CNN senior international correspondents Atika Shubert and Clarissa Ward covering all angles of the investigation into attack.

Let's begin with Clarissa, who is outside the Manchester Arena.

And, Clarissa, we're just learning now that the attacker was apparently a student?

CLARISSA WARD, CNN SENIOR INTERNATIONAL CORRESPONDENT: That's right, Jake; 22-year-old Salman Abedi was a student at the University of Salford.

Salford is a suburb on the outskirts of Manchester. He was reportedly studying business and management. He had attended lectures frequently in his first year, which he had completed, apparently was not attending lectures in his second year, and according to people on campus was not engaged actively socially. Very few people, if any, seemed to know who he even was.

And now authorities are really trying to drill down on whether this young man acted alone or whether there are others still out there. Take a look.

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WARD (voice-over): The investigation into Monday night's deadly bombing that targeted children and teens intensified today.

UNIDENTIFIED MALE: Can you move back down, please?

WARD: Police conducted two raids in Manchester and named the suspected suicide bomber for the first time.

WARD: This home was stormed by armed law enforcement in connection with the investigation. Police say a 23-year-old man has also been arrested in South Manchester in relation to the terror attack that occurred around 10:30 last night.

UNIDENTIFIED FEMALE: Oh, my gosh.

WARD: The blast was heard inside the Manchester Arena just after an Ariana Grande performance, as many parents waited to pick up their children and crowds were streaming out of the exits.

The explosion, outside the venue near the box office, was so powerful, it can be seen and heard on this dash-cam video from a parked car far from the detonation point.

THERESA MAY, BRITISH PRIME MINISTER: The single terrorist detonated his improvised explosive device near one of the exits of the venue, deliberately choosing the time and place to cause maximum carnage. WARD: ISIS has claimed responsibility, but a British counterterror

official tells CNN they have seen no links to known terror groups. President Trump was quick to condemn the attack in his own unique way.

DONALD TRUMP, PRESIDENT OF THE UNITED STATES: I will call them from now on losers, because that's what they are. They are losers.

WARD: Immediately following the blast, thousands fled the scene, leaping over chairs to escape.

UNIDENTIFIED FEMALE: We managed to get through the doors and, how we wasn't crushed to death is a miracle.

UNIDENTIFIED MALE: Obviously, when you have seen children like that as well with blood and who were having to pull nails out of their arms and stuff, and a (INAUDIBLE) little girl's face.

WARD: Police are frantically examining the bomb remnants for clues, while experts say this was more sophisticated than the work of a lone wolf.

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PETER BERGEN, CNN NATIONAL SECURITY ANALYST: How did this bomber learn how to make this? In general, it's, I think, highly unlikely that he just learned about it on the Internet.

WARD: As the United Kingdom reels from its worst attack since 2005, security across the country is stepping up, the prime minister vowing terrorists will not prevail.

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WARD: And, of course, these terror attacks have become all too common across Europe, Jake, but what has authorities specifically concerned in this case is that this man was able to build a bomb. It is a bomb that killed a lot of people.

Bombs tend to be a lot more sophisticated than the sort of cars and knives we have seen, improvised weapons, used by other lone wolves, that leading to suspect that possibly, possibly, I should say, at this stage, a larger network could be at play here -- Jake.

TAPPER: All right. Clarissa Ward, thank you so much.

As investigators are hunting for terror links, British authorities have raided homes and properties over the past 24 hours, including that of the suicide bombers.

CNN's Atika Shubert is in the Fallowfield neighborhood of Manchester, where raids took place.

Atika, are the police still going through the suspect's house? ATIKA SHUBERT, CNN CORRESPONDENT: Well, we still have police here.

You can see they have cordoned off the street, and we have counted about -- well, maybe a little less than a dozen police still in the area there.

Earlier, there were forensics crews going through, taking out documents. And, of course, it started very dramatically with a controlled explosion to gain access into the home.

Now, what we have found by going through public records, speaking with neighbors and friends, is that this is the last known address of the suspect. This is where he was registered as living since 2014.

But we don't have many other details than that. He was a stranger to many of his neighbors, seems to have kept to himself. And we do know, of course, now that he was a university student at Salford University, but didn't live on campus or be part of any student life there, so still very much a mystery as to who this man was.

TAPPER: Atika, do we have any idea what the connection might be between the suicide bomber and the 23-year-old man who was arrested during the raid?

SHUBERT: Police have not given us any details yet, but I did speak to an eyewitness who actually saw the arrest as it happened. It was pretty dramatic. This man was walking down the street near a tram station when a black van with masked and armed police basically swooped in and just efficiently picked him up and put him in cuffs and brought him in, and they said it was very quick and clean, no fuss at all.

But we don't know who the man is or how he's connected at this point.

TAPPER: All right. Atika Shubert in Manchester for us, thank you so much.

We're learning more now about the victims, the children who were killed. And the photographs of their young, excited faces before the attack are just heartbreaking.

Saffie Rose Rousso was just 8 years old. The head teacher at her primary school remembered her as warm and kind, adding -- quote -- "Saffie was quiet and unassuming with a creative flair."

Eighteen-year-old Georgina Callander was an Ariana Grande super fan who had previously met the pop star, as you can see in the photograph. She posted -- quote -- "She was so cute and lovely. I hugged her so tight and she said she loved my bow. I can't get over this. I never will."

John Atkinson of Radcliffe was also killed in the attack. He was a college student with a love for dance who had competed with his local dance studio.

After the frenzied scene last night, some parents are still trying to find their children, desperately calling hospitals and police in hopes that they might just be injured, but alive.

Charlotte Campbell has not been able to find her 15-year-old daughter, Olivia.

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CHARLOTTE CAMPBELL, MOTHER: I love her so much, and I want her home. I need her home. She's my baby, and I miss her so much.

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TAPPER: The pain is just unimaginable.

U.S. officials are beefing up security after the Manchester terror attack. The chair of the Senate Homeland Security Committee will join us next.

Stay with us.

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TAPPER: Welcome back to THE LEAD.

We're back with more on the terrorist attack in Manchester, England, where at least 22 innocent people have been killed at an Ariana Grande concert, many children among those injured and killed.

Joining me now from Capitol Hill is Wisconsin Senator Ron Johnson. He's the chairman of the Senate Homeland Security Committee.

Senator, thanks so much for joining us.

What are you being told about any possible similar threat here in the U.S.?

We will have to let the British authorities investigate this one fully to see if there are any connections, either inside the U.K. or potentially connections to cells here in the United States, but currently we don't -- we have no information in terms of any connection.

Now, I will say, you know, it's depressing. These events are always shocking, but it's depressing they are not surprising. And so we have to remain ever-vigilant.

TAPPER: Senator, was -- the individual who has been named, was he known to U.S. authorities in any way or to British authorities?

JOHNSON: Not that I'm aware of. And, of course, that's what's so difficult about this. We do have lone wolves that are inspired by other Islamic terrorists. There you go. There is one solution. We have to end the caliphate.

We have to destroy ISIS, because as long has they have existed, I think they have existed too long. They continue to inspire this type of activity.

But, you know, once we defeat ISIS, this isn't going away. They have had time to metastasize, evolve, spread. So, this is a long-term, generational struggle, war that we're going to be in, and we need a willing coalition of, you know, the civilized world.

[16:15:01] We need Arabs to speak out and denounce this and try and reform this within Islam, within the Muslim communities and we need to positively engage in American communities. There are so many things that have to be done, but I think staying vigilant is probability number one thing we must remain.

TAPPER: ISIS has claimed responsibility for the attack. Is there any evidence that you know of so far to back up their claim of responsibility?

JOHNSON: None that I'm aware of. It's not surprising that they continue to exist or continue to claim credit, so they continue to inspire additional acts.

TAPPER: Earlier this month, the U.S. State Department released a travel alert warning American citizens about traveling to Europe because of a fear of terror attacks there. Was the attack in Manchester part of that threat stream, do you know?

JOHNSON: Not that I'm aware of. You know, we've always continued to gather intelligence. We've obviously had a ban -- or we did ban the carrying on of smaller electronics, anything larger than a phone. These are real threats, and, you know, certainly our homeland security, our national security personnel are taking these things seriously.

TAPPER: When you see this horrific attack, it's hard for many viewers, many of your constituents not to imagine their own children, their own grandchildren. What are you telling people in your life, your friends, your constituents, who are worried about something similar happening in the U.S.? What precautions should people take other than being vigilant?

JOHNSON: Well, that is the number one thing, and, you know, Jake, I've got to tell a story. A couple years ago, I was briefed by one of the FBI agents that's possibly engaging in Muslim communities, and because of the revelations of Edward Snowden and the assumption that the U.S. federal government knows everything about everybody, they just felt that we knew exactly who might becoming radicalized in their communities. We don't have that perfect information.

So, that's the first thing to understand that the U.S. government has no interest in knowing everything about your life. We don't have perfect information so it's true. If you see something suspicious, you have to say something. And so, it does remains, you know, being vigilant but it also means being supportive of policies to rebuild our military, to secure our homeland, to secure our borders, to make sure that the top priority of the federal government really is national and homeland security.

TAPPER: Why do you think we're seeing more of these attacks in Europe than we are seeing here in the United States?

JOHNSON: Well, one good news item is that in America, Muslim immigrants have assimilated into our culture to a greater extent than they have in Europe. They really have been walled off in a place like France and Brussels and places like the U.K. And so, they have not assimilated those cultures as well as they have assimilated into America. So, that's why we need to continue to engage here in Muslim communities and make sure people do assimilate, that they come to America willing to embrace our culture, you know, take advantage of the opportunities and become part of our society and realize they are going to live by our rule of law, not by Sharia.

TAPPER: Coming up, much more or the terror attack in Manchester. What clues might be hidden in the bomb and then the former CIA director stopped just short of calling the Russian's contact with the President Trump's campaign specific and evident collusion but his comments today raising a lot of questions today. Stay with us.

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[16:22:38] TAPPER: Welcome back.

We have more in our breaking news this hour. An all-out search in England for a possible ISIS terror cell after 22 innocent people were killed, the youngest that we know of, and an 8-year-old girl that were killed at an Ariana Grande concert last night in Manchester, England.

I want to bring in CNN terrorism analyst Paul Cruickshank, national security analyst Juliette Kayyem, and Jane Harman, former House Intelligence Committee member.

Paul, let me start with you. ISIS has claimed responsibility but they haven't offered any proof. What are you hearing?

PAUL CRUICKSHANK, CNN TERRORISM ANALYST: They haven't offered any proof whatsoever describing him as a soldier of the caliphate. And I've just been in touch with a British counterterrorism official that says so far British investigators have uncovered no evidence of any link to an established terrorist group, overseas a terrorist group like ISIS.

Now, it's obviously early on in the investigation. One of the key focuses is on the forensic at the scene of the attack, trying to figure out how sophisticated this device was, what kind of bomb chemicals were in the device and so on, and to try and figure out -- well, could one guy put this together, a young guy in his 20s given his background or could that be a bomb-maker out there, or could that be a sellout there -- Jake.

TAPPER: Juliette, what are we learning about the sophistication of the bomb and possibly how much training that this terrorist had?

JULIETTE KAYYEM, CNN NATIONAL SECURITY ANALYST: Well, this is the quintessential question at this stage because it's not that easy to learn how to make a bomb and get the materials and practice, because in most cases, you want to practice alone, and without being detected. So, all five of those criteria would have had to occur.

So, we're going to start to get more forensics out of the British intelligence and law enforcement authorities to determine where materials were purchased, who were they purchased by and how much was purchased because, of course, in any case like this, if he was a lone wolf then maybe you've limited the threat. The concern is, of course, whether something like this will happen again.

So, I tend to view these investigations as you now have a name and you have a person who -- of the terrorists and now, this investigation is in concentric circles. Some will lead nowhere and some may lead abroad and others may just lead to him which really does limit your capacity to bring a case or something like that.

[16:25:02] TAPPER: And, Congresswoman Harman, obviously, our intelligence officials are doing everything they can to try to find out threats before they can be carried out. What are -- what does the United States and the western world, what are we not doing enough of?

JANE HARMAN, SERVED ON INTELLIGENCE HOMELAND SECURITY CMTES: Well, I was relieved to hear Chairman Ron Johnson say there's no actionable intelligence right now about threats like this to the U.S., but there easily could be these threats.

What are we not doing? First of all, Congress has to multitask. The Russia probe is a big deal, so is health care reform, so is this new budget, which I think is pretty draconian, but so is the possibility that our kids and grandkids could be attacked at a concert this weekend in the United States, and I'm taking my 5-year-old and 9-year- old grandsons to a concert in New York in two days.

And what we could do better is work on technology. They are evolving faster than we are. We need instant counter-messaging in the spaces that they see in the Internet that are recruiting them with videos mostly, not even spoken words anymore to the fight, and the fight can be in London. It can be in Washington, D.C. It can be anywhere.

And our technology companies know how to do this. Congress could, should, encourage lashing up our technical know how with government resources. It's not a zero sum game. It's not privacy versus security. It's both or neither.

TAPPER: Juliet, you heard the congresswoman say she's taking her family to a concert. Arenas here in the United States have stepped up screening to try to prevent anything like this. Is there any way to 100 percent prevent an attack like this? KAYYEM: No, and it's why people like me don't talk about a safe

facility, a safe concert, a safe marathon. You talk about safer, right? So you talk about trying to minimize the risk, put up layered defenses and layered security. But we have to recognize that any event like this, there's going to be a hard part of it, right, a hard target, and there's going to be a soft moment before it. So, you can move out the security ten miles from any concert hall. You're still going to have that point.

So, what we also need to focus on as a mother of three, as someone who is going to attend a lot of concerts this weekend is also to prepare ourselves. We can't just focus on prevention, so that means investments and first responders, trainings and exercises, communicating with children in ways that most of us don't want to and wish we didn't have to. And so, empowering them, especially teenagers who may be off on their own and abroad or at a concert hall, empowering them with the tools because they may not know them intrinsically and we -- and that's our obligation to our children.

TAPPER: And, Paul, let me ask you, as authorities in Manchester and the U.K. are trying to figure out if there's a larger cell, other than going on to the computer and going on to the phone of the terrorist suicide bomber, what can they do to figure this out?

CRUICKSHANK: Well, they have a massive amount of CCTV to go through in terms of the moments before the attack. Where did it come from? They can look at the public transport CCTVs, CCTV in the streets, and try to figure out if he was perhaps given some kind of device in the hours before.

They will obviously be looking through his residence, trying to figure out if that's where the device was built, putting a lot of different pieces together in the hours ahead. They will also be going through their informant network around the U.K. There's a big network that they rely on to try to ask them if they have herd anything about this individual. They will be trying to reach out to family members, trying to account for his travel over the past few years.

We understand from community members in Manchester that he was of Libyan origin. Could he have spent some time in Libya in recent year? Libya, of course, has emerged as a safe haven to some degree for ISIS. These are all urgent questions for the investigation right now.

TAPPER: And there was a warning by the U.S. State Department, Congresswoman, about big public events in Europe, a warning to citizens.

HARMAN: Right.

TAPPER: What do you think that this is part of that threat stream?

HARMAN: Well, it's possible. ISIS has a new publication. It used to have one called Dabiq, except we destroyed their occupation of Dabiq. And so, now, this one is call Romia (ph), after Rome. That is a warning to Europe. But they could easily come here. I was just going to add something Juliette said, and that is about

protection. I head the Wilson Center now and we have done an active shooter exercise. We're located in the Reagan building, which is the largest federal building other than the Pentagon in downtown Washington where a lot of international agencies are, and we think that that kind of attack is possible.

There's perimeter security, but someone could get inside, and I want to protect the workforce. They are all parents and grandparents, too. I mean, yes, we're all vulnerable, but layered security matters and prevention and protection matter and we can do more in the United States, and I really commend our law enforcement effort in the United States.